Consecutive 3rd Tones

It has always been my understanding, that when you have 2 consectutive 3rd tones (i.e. 美女 mei3 nv3 - beautiful woman), that the first word's tone becomes a 2nd tone (i.e. mei2 nv3).

I recently had a conversation with a friend from Beijing and after he pronounced 美女 as mei3 nv3, I asked him about the consecutive 3rd tone rule. He said "No way, never." He said that if you changed the first word to a 2nd tone, then "beautiful woman" would become 霉女 mei2 nv3...."moldy woman"!

I want to beleive him because he is an authority, but it goes against everything I've learned. I would just assume that you would know the meaning based on the context.